[Java] ArrowRecordBatch Body Length Computation Only Accounts for Last Buffer

### Describe the bug, including details regarding any error messages, version, and platform.

Hi Arrow Community,

I believe `ArrowRecordBatch::computeBodyLength` has an implementation bug. It does the following

```
for (int i = 0; i < buffers.size(); i++) {
ArrowBuf buffer = buffers.get(i);
ArrowBuffer layout = buffersLayout.get(i);
size = layout.getOffset() + buffer.readableBytes();

// round up size to the next multiple of 8
size = DataSizeRoundingUtil.roundUpTo8Multiple(size);
}
return size;
```

Since `size` is being assigned and not incremented at first - does this mean computeBodyLength only accounts for the last buffer ?

### Component(s)

Java

從 ArrowRecordBatch::computeBodyLength 開始，檢查緩衝區配置和可讀取的位元組數如何影響傳回值。使用多個緩衝區重現此計算，接著檢查周圍的 Java 程式碼或測試，以確認預期的主體長度語義。完成的標準是：計算涵蓋每個緩衝區，並保留所需的 8 位元組對齊。

由索引模型根據 Issue 內容生成。